(1) Field of the Invention
The present invention relates to the field of emergency lubrication for transmission gearboxes, in particular power transmission main gearboxes for a rotary wing aircraft.
The present invention relates to an emergency lubrication device of simplified architecture for a mechanical system. The emergency lubrication device is particularly adapted to lubricating a power transmission main gearbox of a rotary wing aircraft.
(2) Description of Related Art
A mechanical system generally includes rotary elements, such as shafts and bearings, together with power transmission elements and speed increasing or decreasing elements, such as pinions and/or gears. It is then essential for proper operation of the mechanical system to lubricate and cool these elements, e.g. with oil. The lubrication is generally provided by a lubrication circuit and its main functions are to limit wear and heating of these elements of the mechanical system and consequently to lengthen its lifetime. Without such lubrication, the operation of the mechanical system can rapidly become degraded, or even impossible.
As a result of the mechanical system being lubricated, the oil flowing in the lubrication circuit can sometimes become very hot and it is then cooled in a cooling circuit that is generally outside the mechanical system prior to being usable once more for lubricating the mechanical system. The cooling circuit includes a heat exchanger, e.g. an oil/air heat exchanger.
The cooling circuit outside the mechanical system constitutes a vulnerable portion of a lubrication circuit of the mechanical system when it comes to leaks. The cooling circuit has pipes, numerous connections, and the heat exchanger. The cooling circuit is then subjected to thermal stresses, such as a large temperature difference between the temperature of the oil and the outside temperature, and to vibratory stresses generated by the mechanical system and/or a vehicle using the mechanical system. Furthermore, the cooling circuit is arranged outside the mechanical system. In particular, when the mechanical system forms part of an aircraft, the cooling circuit is situated outside the mechanical system of the aircraft, e.g. under a cover. Nevertheless, it can be exposed to impacts against birds or indeed ice, for example. As a result, one or more leaks may appear in the couplings and the pipes and also in the heat exchanger, which leaks are caused essentially by these vibratory and thermal stresses.
Such leaks generally allow the mechanical system to continue to be lubricated, but for a duration that is limited. Specifically, it can happen that all of the oil stored in the lubrication circuit, e.g. in a tank, becomes discharged to the outside of the lubrication circuit via such leaks. Such leaks may possibly be detected by a drop in the pressure of the oil in the lubrication circuit.
Furthermore, a lubrication circuit also has a pressure generator, such as a pump, for feeding the lubrication circuit with oil, and thus enabling the oil to flow through the lubrication circuit. In the event of the pressure generator failing, the flow of oil is interrupted, and consequently the lubrication of the mechanical system is likewise interrupted immediately.
In the event of this lubrication being lost, damage can therefore appear rapidly in the operation of the mechanical system. By way of example, the consequence of such damage taking place in a mechanical system forming part of a motor vehicle, may be the vehicle coming to a stop immediately, or else coming to a stop after the oil in the lubrication circuit has been consumed.
In contrast, if the mechanical system is a power transmission main gearbox of a rotary wing aircraft, such damage to the lubrication circuit of the power transmission main gearbox can then have consequences that are catastrophic, such as an emergency landing or indeed a crash of the aircraft.
In order to mitigate such consequences, a mechanical system may include an emergency lubrication circuit. In the event of the main lubrication circuit being out of operation, such an emergency lubrication circuit makes it possible to provide lubrication, at least for essential members of the mechanical system, in order to ensure that the mechanical system continues to operate. For safety reasons, it is then preferable for the aircraft to operate at reduced power in order to limit stresses on the mechanical system. The emergency lubrication circuit thus makes it possible, e.g. when the mechanical system is a power transmission main gearbox of an aircraft, for the mechanical system to continue to operate, and consequently for the aircraft to continue to operate, in order to reach a landing site. Such an emergency lubrication circuit thus improves the safety of the aircraft.
An emergency lubrication circuit may be arranged in parallel with a main lubrication circuit, as described in Document U.S. Pat. No. 8,230,835. Each lubrication circuit then has its own pump, but uses the same oil tank. Nevertheless, although that emergency circuit enables a mechanical system to be lubricated sufficiently in the event of a failure of the main circuit, it is in fact used only rarely. As a result, the emergency circuit constitutes an on-board weight that is rarely used.
The emergency lubrication circuit may also have a tank that is independent from the main lubrication circuit, as described in Document FR 2 826 094. Furthermore, in that Document FR 2 826 094, the lubrication system includes a source of gas under pressure capable firstly of pressurizing the lubrication liquid contained in the tank, and secondly of causing the lubrication liquid to be sprayed in the form of a mist.
Furthermore, the zones where leaks appear in the main lubrication circuit are frequently to be found in the heat exchanger and its couplings. As a result, in order to avoid such leaks also appearing in the emergency lubrication circuit, the emergency circuit does not have a heat exchanger. As a result, the oil flowing due to the emergency circuit is then not cooled. Consequently, the emergency lubrication circuit can be used for only a limited duration in order to avoid the oil reaching a temperature that is too high.
That emergency circuit is generally put into operation automatically as a result of detecting a loss of pressure in the main circuit as a result of a failure of the pump of the main circuit or indeed as a result of a leak from the main circuit. The emergency lubrication circuit may also be put into operation manually by an operator.
In order to reduce any risk of a failure in the main lubrication circuit coming from the emergency circuit, the emergency circuit is generally provided with a bypass system for the purpose of shutting off flow in the pipes of the emergency system so long as there is sufficient oil pressure in the main lubrication circuit. Thus, in the event of a leak in the emergency lubrication system, the emergency system is no longer operational, but it does not impede operation of the main lubrication circuit.
The drawback of the bypass system is that it leaves open the possibility of a dormant (hidden) failure in the emergency circuit, with this failure being detected only at the moment when the emergency circuit is put into operation. Under such circumstances, this is a major anomaly that might be mission critical on a rotary wing aircraft.
For certain applications of rotary wing aircraft, the power transmission main gearbox does not have an emergency lubrication circuit properly speaking, but rather two lubrication circuits that are identical and independent. Each lubrication circuit has its own pump and its own heat exchanger. Nevertheless, those two circuits both use the same oil tank in common, which is constituted by the casing of the power transmission main gearbox. As a result, in the event of a leak from either of the lubrication circuits, the casing of the power transmission main gearbox will sooner or later become empty and lead to a failure of the complete lubrication system.
Some lubrication systems have an emergency tank, sometimes mounted directly inside the mechanical system for lubricating, as described in Documents EP 2 505 878 and US 2007/0261922. The emergency tank is located above members that it is essential to lubricate and that are fed permanently by the lubrication circuit. Oil then flows under gravity and continuously from the emergency tank to such essential members. Specifically, in the event of a failure of the lubrication circuit, the emergency tank is no longer fed, but it serves to lubricate such essential members for a limited time that corresponds to consuming the oil contained in the emergency tank.
Furthermore, according to Document US 2007/0261922, an additive may be added to the oil in the emergency tank after detecting a failure of the main lubrication circuit. The additive serves to enhance the characteristics of the oil so as to improve its effectiveness and the duration for which the emergency lubrication circuit constituted by the emergency tank can operate.
In addition, the device described in Document FR 2 685 758 also has an emergency tank feeding an oil distribution system via tubing. The emergency tank may be pressurized and opening a valve allows oil to flow in the tubing. It is also possible for the emergency tank to not be pressurized, with the tubing being fed via a pump. The emergency tank can thus be positioned without any particular constraint relative to the essential members that are to be lubricated. In the event of a failure of the main lubrication circuit, the valve is opened or the pump is started, either manually or automatically, depending on a threshold pressure value or on the temperature of the oil. The essential members are lubricated for a limited time corresponding to consuming the oil contained in the emergency tank.
Furthermore, Document WO 2008/091341 describes a lubrication device having an external circuit serving solely for cooling oil and an internal circuit serving for lubricating rotary elements of the mechanical system. In the event of detecting a leak and a drop in the level of lubrication liquid in the casing, air inside the casing is expelled to outside the casing in order to create suction inside the casing, thereby limiting the flow rate of the leak. The internal circuit can then continue to lubricate the rotary elements for a limited length of time, so long as the casing contains lubrication liquid.